‘Uthman bin al-Aswad said: I asked Mujahid (may Allah have mercy on him) about a woman who gets her menses: Is it permissible for her husband to have intercourse with her before she performs ghusl (ritual bath)? He replied: No, as long as prayer is not permissible (intercourse is also not permissible).
Hadith Referenceسنن دارمي / من كتاب الطهارة / 1118